The Chi-Rho is a symbol for the Greek ΙΗΣΟϒΣΧΡΙΣΤΟΣ (Jesus Christ) constituted by the abbreviation of the first letters of the two words, I and CR (C or chi, and R or rho) respectively. The first dated use is 269. In the pre-Constantinian use, the chi and iota were combined, almost always as part of an inscription with έν (in Christ) or with δο[symbol omitted]λος (servant of Christ). The Chi-Rho form stood for I and CR (Jesus and Christ), but the symbol had been in use among the pagans as an abbreviation for many words beginning with CR, such as χρόνος (time) and χρυσός (gold). However, the vision of constantine i in 312, as reported by lactantius (De morte 44) and eusebius of caesarea in the Vita Constantini, led to the use of the symbol of the name or cross of Christ on the soldiers' shields, on the labarum, and on coins, apparently in the Chi-Rho form, as well as in the so called Egyptian tau form; alternate forms with the vertical axis placed at an angle and the loop of the rho reversed became common also.
The earliest Christian employment of the Chi-Rho monogram dates from the reign of Constantine. During the 4th and 5th centuries its use on coins, on the exterior and interior of churches and basilicas, as well as on sarcophagi and funeral monuments in the catacombs and cemeteries, became widespread. It was used also as a decorative emblem on glasses and cups, on the exterior of homes, particularly in Syria, and on medals, rings, furniture, and utensils. St. john chrysostom mentions its use in epistles [Patrologica Graeca, ed. J. P. Migne, (Paris 1857–66) 62:364]. During this period it was likewise used in combination with other symbols. It is found flanked with the alpha and omega (A and Ω) standing for Christ in the midst of the divine eternity, or with the A and Ω pendant from the cross-bar. In Gallic inscriptions and decorations the Chi-Rho is almost always surrounded by a wreath or palm or other leaves, symbolizing the Lord's victory; and occasionally the Chi-Rho is found in the midst of an N, meaning Christ conquers. During the Middle Ages it appeared at the beginning of charters; Merovingian scribes distorted it with flourishes. The imperial chancery used it until c. 1200. It appeared in papal documents frequently until the pontificate of leo ix (d.1054); after the time of gregory vii (d. 1085) the chrismon was used only in private charters. In the late Middle Ages the Chi-Rho symbol was gradually replaced by the combination IHS, derived from the first three Greek letters of the name Jesus. By popular etymology the IHS was interpreted as In hoc signo (In this sign, namely, the sign of the cross of Christ), or as Jesus hominum Salvator. The Chi-Rho was used, particularly in the East, as an amulet to ward off sickness or danger from evil spirits.
Bibliography: i. sauer, Lexikon für Theologie und Kirche 2:1177, with bibl. f. cabrol, Dictionnaire d'archéologie chrétienne et de liturgie, ed. f. cabrol, h. leclercq, and h. i. marrou, 15v. (Paris 1907–53) 1.1:1–25. h. leclercq, ibid. 3.1:1481–1534. l. traube, Nomina sacra (Munich 1907). v. gardthausen, Das alte Monogramm (Leipzig 1924). a. wrede, in Handwörterbuch des deutschen Aberglaubens, ed. h. bÄchtold–stÄubli, 10 v. (Berlin 1927–42) 2:76–82.
c. m. aherne]
"Chi-Rho." New Catholic Encyclopedia. . Encyclopedia.com. (August 14, 2018). http://www.encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/chi-rho
"Chi-Rho." New Catholic Encyclopedia. . Retrieved August 14, 2018 from Encyclopedia.com: http://www.encyclopedia.com/religion/encyclopedias-almanacs-transcripts-and-maps/chi-rho